Method of separating a mixture of plastics comprising at least three components using electrostatic techniques

ABSTRACT

Plastic mixtures are split up into the individual components by a process combining density separation with electrostatic separation, and includes a special surface treatment which takes place prior to the electrostatic separation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for the separation of plasticparticles of a plastic mixture of plastics of a chemically differenttype which partly have an overlapping and partly a different densityrange, e.g. polyethylene (PE), polyethylene terephthalate (PET),polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC).

2. The Prior Art

Such different types of plastic occur as waste, for example whendifferent one-way bottles are mixed. For example, still waters arepredominantly filled in 1.5-liter PVC-bottles, whereas other beveragesare sold in so-called PET-bottles. In Western Europe alone, 1.4 billionPET-bottles are manufactured annually. The bottles have, as a rule, apolyethylene screw cap, whereby the PET-bottles may have a bottom partmade of polyethylene as well. Direct recycling of the mixed bottleplastics is not possible because PET melts only at 260° C., whereas PVCdecomposes with separation of HCl already above the softeningtemperature of 160° C. There are, therefore, no appreciable recyclingpossibilities, so that the waste plastics have not been collectedheretofore but eliminated via the household refuse, i.e., they arefinally incinerated or deposited.

Furthermore, it is not possible, as a rule, to achieve any profits formixed PVC-containing plastics. The fact is rather that the reuserfrequently demands a credit that is oriented on the dumping costs saved.

As opposed to the above, there is a market for purely sorted recyclingplastics since long, whereby the prices are oriented on the prices fornew material. Up to 60% of the new material is achieved for recyclingmaterial depending on the quality. Thus there is much interest inprocesses for the separation of mixed plastics.

The processes known from the state of the art for the separation ofplastic particles of plastics of a chemically different type operatewith plants separating according to the density, for examplehydrocyclones. Said process, however, fails in connection with plasticsthat are in the same density range such as, for example, PET (densityabout 1.37 to 1.38 g/cm³) and PVC (density about 1.38 g/cm³). However,the separation of polyethylene (PE) from the other two plastic types PETand PVC is possible because of the different density of 0.95 g/cm³. Theseparation of plastics that are in the same density range can be carriedout, for example electrostatically.

It is known from DE-PS 30 35 649 to separate plastics electrostaticallyin a free-fall separator.

However, it has been found that in the separation of a plastic mixturewith three or four different types of plastics, thus for example PE,PET, PS and PVC with one of said processes, a large quantity of mediummaterial is collected, or that the deposits on the respective electrodehave only an insufficient degree of purity. Furthermore, the mediummaterial has a high component of at least one of the plastics used.

SUMMARY OF THE INVENTION

The invention, therefore, has the object of creating a process of thetype specified at the beginning in which several components of a plasticmixture even of similar or the same densities can be safely separatedfrom one another. This object is achieved in that the separation takesplace in at least two steps, whereby in a first step, the plasticparticles having a different density range are separated from eachother, and whereby in a second step, the plastic particles with the samedensity range are separated. In this connection, the plastic particlesare advantageously separated in the first step according to theprinciple of density separation, whereby the density of the separationliquid is selected in such a way that it falls in the field of thegreatest density difference between the individual plastic types of theplastic mixture; advantageously, the density of the separation liquid isadjusted in this connection between 1.0 and 1.3 g/cm³. The densityseparation can take place in this connection by means of a hydrocycloneas well. If necessary, the separation according to the density takesplace not only in one step but in several ones if several types ofplastic with a different density are to be separated.

Furthermore, it has been found that it is possible to achieve through asurface treatment of the plastic particles of the plastic mixture animproved triboelectric charging in the sense of a higher charge density.

According to an advantageous feature of the invention, the chemicaltreatment of the surface of the plastic particles of the plastic mixturetakes place in that the separation liquid is selected in such a way thatit is in the basic range (pH about 10 to 12) or in the acid range (pH of2 to 4). Particularly advantageous results are obtained if theseparation liquid is a salt solution of which NaCl is the maincomponent. In addition to the NaCl in the salt solution, K-, Mg- and SO₄-ions may be present in the salt solution as well, i.e., because of thedesired composition of the salt solution it is possible to use a saltsolution as formed as a waste product in the production of potash inpotash mining. An enhanced triboelectric charging is particularlyachieved also if, after the density separation, the separation liquid iswashed out of the plastic mixture by water. In the course of densityseparation or of the subsequent cleaning of the plastic mixture withwater, the plastic particles having a size of under 10 and preferablyabout under 6 mm can be cleaned from paper residues or beverageresidues. However, such cleaning is possible also in a washing processcarried out prior to the density separation, for example in a washingmill or in a turbo-washer. After the washing, a drying of the plasticmixture takes place, whereby prior to the actual drying, the watercontent of the plastic mixture is reduced by a dehydration aggregate,e.g. a centrifuge, to a residual water proportion of under 2%.

In the following, the plastic mixture is subjected to a thermaltreatment at 30° to 100° C. over a time period of at least 5 minutes;this measure, too, serves for achieving a higher charge density of theindividual plastic particles. This is seemingly explainable in that dueto the thermal treatment in the aforementioned temperature range, achange occurs in the surface of the plastic particles. The surfacetreatment can be achieved both chemically and through heat, or throughboth types of treatment.

According to another advantageous feature of the invention, an organicsubstance, in particular fatty acid is added to the plastic mixture inan amount of about 10 to 50 mg/kg plastic mixture. The addition of fattyacid serves for the conditioning of the plastic particles, also with theobjective of obtaining in the subsequent triboelectric charging a highercharge density of the individual particles. This treatment, too, cantake place alone or in combination with the chemical or thermaltreatment of the plastic particles.

It has been found that with plastic particles pretreated in said way,only field intensities of 2 to 3 KV/cm have to be maintained in thefree-fall separator itself.

As opposed to the above, the free-fall separator operates in connectionwith the known process with a field intensity of 3 to 4 KV/cm, whichposed the danger of spray discharges. Spray discharges may cause anignition of the plastic mixture in the free-fall separator.

The triboelectric charging itself takes place, for example in afluidized-bed dryer, or in a spiral worm of adequate length, or also bypneumatically conveying the plastic mixture over a certain distance. Asmarginal conditions it is necessary to maintain in the triboelectriccharging temperatures of about 15° to 50° C., preferably 20° to 35° C.,and a relative humidity of the ambient air of 10 to 40%, preferably 15to 20%. The triboelectric charging of the plastic particles themselvestakes place in the known way by intimately contacting of the particleswith one another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram relating to Example 1;

FIG. 2 shows a flow diagram relating to Example 2; and

FIG. 3 shows a flow diagram relating to Example 3.

The process according to the invention is explained on the basis of thefollowing examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1: Separationof a mixture of beverage bottles is shown in FIG. 1

The mixture of beverage bottles used had the following composition:

76.9% PET

19.8% PVC

2.1% PE

1.2% paper/dirt.

The bottle mixture was fed into a wet-operating cutting mill and, underaddition of water, crushed to a particle size of under 6 mm. The dirtsolution, which also contained paper, was drawn off. Subsequently, thematerial was vigorously stirred in a washer, which cleaned the surfacesand prepared the latter for the later electrostatic separation.

For separating the polyolefins (PE), the substances was admitted into ahydrocyclone. The resulting PVC-PET--mixture was separated from theliquid on a vibration screen, centrifuged, and dried for 6 minutes in afluidized-bed dryer at 70° to 100° C.

In the fluidized bed, any last paper residues that might still bepresent can be discharged with the exhaust air and separated from theexhaust air by means of a cyclone. The predried material wassubsequently contacted in another fluidized-bed dryer for another 3minutes at 30° C. and at the same time charged.

The material draining from the fluidized bed was continuously admittedto a separating plant consisting of two separators. A PET-concentratewith 99.4% PET is obtained already in the preliminary separation; thePVC-concentrate with a PVC-content of 82.3% was conveyed to theafter-separation separator by means of a spiral worm, whereby theselective charging of the plastic particles developed.

In the after-separation separator, the preconcentrate so charged wasseparated into a high-percent PVC-concentrate, a medium-materialfraction, and a deconcentration fraction containing about 53% PET. Thelatter together with the medium material of the preseparation wasrecycled into the fluidized bed for new charging.

All in all, it was possible to separate the plastic mixture into

a PVC-fraction with a degree of purity of 99.3% PVC,

a PET-fraction with a degree of purity of 99.4% PET, and

a PE-fraction with a degree of purity of 97.6 PE;

the yield (absolute quantity)--based on the bottle mixtureused--consisted of:

94.6% PET

96.2% PVC

89.7% PE.

EXAMPLE 2: Separation of a PE/PP/PS/PVC plastic mixture is shown in FIG.2

The used mixture of used plastic articles contained four of the mostcommonly used mass plastics in the following composition:

45.7% PE

20.1% PP

17.5% PVC

14.9% PS

1.8% residual substances.

100 kg of said mixture was first completely crushed on a cutting mill toa grain size of under 6 mm. The shred mixture was fed into a washer andstirred with fresh water. The washed material was transferred into aflotation basin filled with water, whereas the dirt solution wasdiscarded. The light fraction containing the polyolefins was skimmedoff, whereas the heavy fraction containing the PVC and PS was sucked offat the bottom of the basin. Both fractions were predehydrated by meansof centrifuging.

The PP/PE-fraction was fed into a fluidized-bed dryer and dried for 6minutes at 80° C. A fatty acid mixture C8-C12 was sprayed onto thedraining material in an amount of 50 g/t, and fluidizing was carried outin another fluidized-bed dryer for 3 minutes at 30° C. The mixtureflowing from the fluidized bed was continuously fed into a free-fallseparator. The medium material of said separation was continuouslyrecycled into the second fluidized-bed dryer.

The electrostatic separation of the light fraction supplied thefollowing result:

    ______________________________________                                                       Analysis      Yield                                                   Quantity                                                                              (degree of purity                                                                           (in % of the                                            kg      in %)         charge)                                          ______________________________________                                        PE-fraction                                                                            44.1      96.6          92.2                                         PP-fraction                                                                            20.6      88.5          90.7                                         ______________________________________                                    

The heavy fraction was transferred into a fluidized-bed dryer with aconnected cooler, dried in the heating zone for about 6 minutes at 80°C., and fluidized in the cooling zone for about 3 minutes at 30° C. Theelectrostatic separation, with recycling of the medium material in thiscase too, supplied the following result:

    ______________________________________                                                       Analysis      Yield                                                   Quantity                                                                              (degree of purity                                                                           (in % of the                                            kg      in %)         charge)                                          ______________________________________                                        PVC-fraction                                                                           17.3      97.1          95.9                                         PS-fraction                                                                            14.8      94.3          93.7                                         ______________________________________                                    

EXAMPLE 3: Separation of a PE/PS/PET/PVC-mixture into the individualcomponents is shown in FIG. 3

The used mixture of used plastics had the following composition:

46.8% PE

29.8% PS

12.2% PVC

10.1% PET

1.1% dirt.

100 kg of said mixture was first completely crushed in a cutting mill toa grain size of under 6 mm. The shred mixture was fed into a washer andstirred with fresh water. The washed material was filled in a flotationbasin filled with potash waste liquor with a density of 1.2 g/cm³.

The light fraction containing PE and PS was skimmed off, whereas theheavy fraction containing PVC and PET was sucked off at the bottom ofthe basin. Both fractions were predehydrated on a swing screen, washedwith fresh water, and subsequently predehydrated on centrifuges to anadhering moisture of 2%. The salt-containing waste waters collected inthe density separation and predehydration can be recycled into thepotash dissolving operation for treatment. Both fractions were fed intoseparate fluidized-bed dryers equipped in each case with a heating and acooling zone. In the hot zone, the materials were heated to 80° C.,whereby the dwelling time came to about 6 minutes, whereas the coolingzone connected downstream was operated with unheated air.

The materials flowing from the fluidized beds were fed intoelectrostatic free-fall separators, whereby the collected mediummaterials were recycled into the fluidized beds.

The electrostatic separation of the light fraction supplied thefollowing result:

    ______________________________________                                                       Analysis      Yield                                                   Quantity                                                                              (degree of purity                                                                           (in % of the                                            kg      in %)         charge)                                          ______________________________________                                        PE-fraction                                                                            43.8      95.6          93.5                                         PS-fraction                                                                            27.7      92.4          92.9                                         ______________________________________                                    

The following result was obtained in the electrostatic separation of theheavy fraction:

    ______________________________________                                                       Analysis      Yield                                                   Quantity                                                                              (degree of purity                                                                           (in % of the                                            kg      in %)         charge)                                          ______________________________________                                        PVC-fraction                                                                           12.6      93.9          96.6                                         PET-fraction                                                                           9.2       97.1          88.0                                         ______________________________________                                    

We claim:
 1. Process for the separation of crushed plastic particles ofa plastic mixture of plastics of a chemically different type, whichpartly have an overlapping density range and partly a different densityrange selected from the group consisting of polyethylene (PE),polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS),and polyvinyl chloride (PVC), comprising the steps offirst separatingthe plastic particles having a different density range according to theprinciple of density separation; and second separating the plasticparticles with the same density range via a free-fall separator;subjecting the plastic mixture to a surface treatment; and triboelectriccharging the plastic mixture.
 2. Process according to claim 1, furthercomprising the step of providing a separation liquid;wherein the densityof the separation liquid is selected in such a way that the density isbetween the greatest density and the least density of the individualplastics.
 3. Process according to claim 1, further comprising the stepsofwashing the separation liquid out of the plastic mixture by waterafter the first step of density separation.
 4. Process according toclaim 3, further comprising the steps ofreducing the plastic mixture bya dehydration aggregate, to a residual water proportion of under 2%: anddrying the plastic mixture after the washing.
 5. Process according toclaim 4 further comprising the steps ofsubjecting the dried plasticmixture to a thermal treatment at 70° to 100° C. over a time period ofat least 5 minutes.
 6. Process according to claim 2, further comprisingadjusting the density of the separation liquid to values between 1.0 and1.3 g/cm³.
 7. Process according to claim 1, comprising triboelectriccharging of the plastic mixture at a temperature of 15° to 50° C., andat a relative humidity of the ambient air of 10 to
 40. 8. Processaccording to claim 1, comprising triboelectric charging the plasticmixture by loading in a fluidized-bed dryer.
 9. Process according toclaim 1, further comprising passing the plastic mixture through a spiralworm of sufficient length.
 10. Process according to claim 1, furthercomprising conveying the plastic mixture pneumatically over a specifieddistance.
 11. Process according to claim 1, further comprising operatingthe free-fall separator at a field intensity of 2 to 3 KV/cm. 12.Process according to claim 7, wherein the triboelectric charging of theplastic mixture occurs at a temperature of 20° C. to 35° C. and at arelative humidity of the ambient air of 15% to 20%.
 13. Process for theseparation of plastic particles of a plastic mixture of plastics of achemically different type, which partly have an overlapping densityrange and partly a different density range, selected from the groupconsisting of polyethylene (PE), polyethylene terephthalate (PET),polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC),comprisingin a first step, the plastic particles having a differentdensity range are separated from one another according to the principleof density Separation and; in a second step, the plastic particles withthe same density range are separated, wherein the density of aseparation liquid is selected in such a way that the density is betweenthe greatest density and the least density of the individual plastics,and wherein the separation liquid is selected in such a way that it iseither in the basic range having pH about 11 to 12 or in the acid rangehaving pH about 2 to
 4. 14. Process according to claim 13, wherein theseparation liquid is a salt solution of which the main component isNaCl.
 15. Process according to claim 14, wherein K-, Mg- and SO₄ -ionsare additionally present.
 16. Process for the separation of plasticparticles of a plastic mixture of plastics of a chemically differenttype, which partly have an overlapping density range and partly adifferent density range, selected from the group consisting ofpolyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP),polystyrene (PS), and polyvinyl chloride (PVC), comprisingin a firststep, the plastic particles having a different density range areseparated from one another according to the principle of densityseparation; in a second step, the plastic particles with the samedensity range are separated; washing the separation liquid out of theplastic mixture by water after the first step of density separation;reducing the plastic mixture by a dehydration aggregate to a residualwater proportion of under 2% prior to drying; drying the plastic mixtureafter washing; and adding an organic substance to the plastic mixture.17. Process according to claim 16, wherein the organic substance is afatty acid.
 18. Process according to claim 17, further comprising addingthe fatty acid in an amount of 10 to 50 mg/kg plastic mixture. 19.Process according to claim 16, wherein the plastic particles of theplastic mixture have a size of under 10 mm.
 20. Process according toclaim 19, wherein the plastic particles of the plastic mixture have asize under 6 mm.